The last time the Charming’s saw Maleficent she roasted some guards because they were in her way to the Tree of Knowledge. I can totally buy Snow making a huge judgement call on any of her spawn.
Here’s the thing though…Snowing believe themselves to be good, yet still believe their spawn could be evil. So if you just take that logic and flip it, why should they assume that just because Maleficent is evil, that her spawn would turn out the same as her? They’re operating with the mindset that their child might not follow in their footsteps, but obviously, without a doubt, Maleficent’s child will absolutely turn out just like its mother. Hypocritical much?